1. Field of the Invention
The invention relates to a pixel driving circuit, and in particular to a pixel driving circuit with threshold voltage and power supply voltage compensation.
2. Description of the Related Art
Organic light emitting diode (OLED) displays utilizing organic compounds as a lighting material are common in flat displays, providing desired small size, light weight, wider viewing angle, high contrast ratio and high response speed.
Active matrix organic light emitting diode (AMOLED) displays are currently emerging as the next generation of flat panel displays. Compared with active matrix liquid crystal displays (AMLCD), the AMOLED display has many advantages, such as high contrast ratio, wide viewing angle, thin module without backlight, low power consumption, and low cost. Unlike the AMLCD display, which is driven by a voltage source, an AMOLED display requires a current source to drive an electroluminescent element. The brightness of the electroluminescent element is proportional to the current conducted thereby. Variations in current level have a great impact on brightness uniformity of an AMOLED display. Thus, the quality of a pixel driving circuit is critical to the quality of an AMOLED display.
FIG. 1 shows a conventional 2T1C (2 transistors and 1 capacitor) pixel driving circuit 10 in an AMOLED display. Pixel driving circuit 10 comprises transistors Mx and My, electroluminescent element EL and capacitor Cst. When signal Scan turns on transistor Mx, data signal shown as Vdata in the FIG. 1 is loaded into a gate of p-type transistor My and stored in capacitor Cst, providing a constant current driving electroluminescent element EL to emit light. Typically, in an AMOLED display, a current source is implemented by a P-type Thin film transistor (TFT) (My in FIG. 1) gated by data signal Vdata and having source and drain connected to Vdd and the anode of electroluminescent element EL, respectively, as shown in FIG. 1. The brightness of electroluminescent element EL with respect to Vdata therefore has the following relation.Brightness ∝ current ∝ (Vdd−Vdata−Vth)2 Where Vth is a threshold voltage of transistor My and Vdd is a power supply voltage.
Since there is typically a variation in Vth for a LTPS type TFT due to a low temperature polysilicon (LTPS) process, non-uniform brightness can occur in an AMOLED display if threshold voltage Vth is not properly compensated. Moreover, a voltage drop in the power line also causes the brightness non-uniformity problem. To overcome such problems, implementation of a pixel driving circuit with threshold voltage Vth and power supply voltage Vdd compensation to improve display uniformity is required.